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North Korea has made no secret of its goal to produce an inter-continental ballistic missile capable of hitting the United States mainland, giving it the means to send a nuclear warhead to its arch-enemy.

Kim Jong Un's rocket scientists are thought to be several years from being able to do this, instead concentrating on intermediate-range missiles that can reach only as far as Guam.

But now some analysts are asking: Did North Korea just try to launch two long-range missiles?

"We think it is important that people consider the possibility that this was a KN-08 test," said Jeffrey Lewis, director of the East Asia non-proliferation programme at the Middlebury Institute of International Studies in California, referring to the inter-continental ballistic missile, or ICBM, by its technical name.

North Korea conducted two missile tests this month, on October 9 and 19. Both took place at an airbase in Kusong on North Korea's west coast, on the other side of the country from the usual Musudan test site near Wonsan, on the east coast.

The United States Strategic Command said both tests were "presumed" to be of Musudan missiles, and South Korean military officials have said the same. The Musudan is technically capable of flying as far as 3860km, putting Guam within range and almost reaching Alaska.

These would mark the seventh and eighth Musudan tests this year. Only one, in June, was a success, flying about 400km and reaching a surprisingly high altitude.

But, after poring over satellite photos enhanced with a near infra-red band of light, Lewis and his fellow experts at MIIS think there's an even chance that the launches were of ICBMs.

"We're not fully persuaded that it was a Musudan," said Lewis, noting that Strategic Command, known as StratCom, twice mis-identified three missiles that North Korea launched in September.

"We still think people are being too quick to jump to the conclusion that this was a Musudan. Even if it's only 50/50, being shocked half of the time is still bad," he said.

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Looking at before-and-after satellite imagery from Planet Labs, a San Francisco-based imaging company, Lewis and his colleagues found burn scars after each missile firing, showing where the hot exhaust scorched the pavement and grass. Near infra-red light can discriminate between burned and untouched parts vegetation.

"The first scar is really big. That is consistent with a catastrophic failure," he said, noting that it was possible the explosion melted the launch vehicle on which it was mounted. "The second burn scar is small, like the missile flew some distance before whatever went wrong."

These big burn scars are much bigger than what had been seen after Musudan tests, and the fact that they happened on the other side of the country added to suspicions about the kind of missile being tested.

Other analysts said it was possible, if not probable, that the tests were of ICBMs.

"While this was most likely a Musudan test, the possibility of a KN-08 cannot be ruled out," said John Schilling, an aerospace engineer who frequently writes about the North's missiles. Schilling expects North Korea's road-mobile ICBMs to reach operational status early in the next decade - perhaps within five years.

"It seems most likely that the latest test was an attempt to test the Musudan from an operational launch facility rather than a test facility," he said, explaining the difference in test location.

But, while noting that StratCom should be able to distinguish between a Musudan and a much larger ICBM, he also pointed out that the US agency had misidentified the three missiles launched last month. StratCom first called them short-range Rodongs, then medium-range Musudans. They turned out to be extended-range Scud missiles.

"I do not place high confidence in their ability to distinguish a KN-08 from a Musudan," Schilling said. "And I do not think anyone should have high confidence in their ability to predict North Korean behaviour regarding missile testing."

Joseph Bermudez, chief analytics officer at AllSource Analysis, a Denver-based consultancy, also said the likelihood was that the missiles were medium-range Musudans.

"Given their concentration on perfecting the Musudan, I would think they were testing a Musudan," he said, saying that the test site could have been moved simply to make it harder for intelligence agencies to monitor the activity.

Although all but one Musudan test has failed, North Korea has shown rapid advances with its missile programme this year. It successfully launched a ballistic missile from a submarine in August and launched what it said was a satellite in February. That launch was widely viewed as cover for an ICBM test.

Japan's Asahi television network, citing an unnamed North Korean military source, reported this month that Kim had ordered ICBM development to be finished by the end of this year. A spokesman for North Korea's space agency last week pledged that the state would continue working on its "peaceful" satellite programme.

Regardless of whether this latest test was of medium- or long-range missiles, analysts agree that they are cause for concern.

"What's more concerning is not an individual test or two individual tests, it's that they're approaching their missile development in a very pragmatic way," Bermudez said.

"What they're doing is exactly right. They are testing, and they are testing often," he said. "This is the way you really learn how to develop a ballistic missile, and that's what worries me."

In the United States, there is growing recognition of the increased threat posed by the North Korean regime.

James Clapper, the Director of National Intelligence, said yesterday that the United States had to assume that North Korea would be able to mount a nuclear warhead on a missile capable of reaching the US west coast.

"We ascribe to them the capability to launch a missile that would have a weapon on it to reach parts of the United States, certainly including Alaska and Hawaii," he said at a speech at the Council on Foreign Relations.

"They could do it. We have to make the worst-case assumption here," he said.

Clapper also said that trying to convince North Korea to give up its weapons was now probably a "lost cause" given that the programme was its "ticket to survival."

Earlier this month, Jami Miscik, previously a top analyst at the Central Intelligence Agency, warned against underestimating North Korea's capabilities.

"The fact these rocket launches are failing shouldn't give anyone comfort," she said at Fortune's Most Powerful Women summit, according to the magazine. "They fail and they fix that thing, and it might fail for another reason, but they're advancing in terms of their capability."